Continuous selftest for inertial sensors at 0 hz

ABSTRACT

A sensor with continuous self test ( 101 ). An exemplary inertial sensor ( 106 ) may include one or more self test electrodes ( 208, 210 ) so that one or more test signals ( 402, 404 ) may be applied to the electrodes ( 208, 210 ) during normal operation of the sensor. Normal sensor output may be read and stored ( 316 ) during normal operation, when self test signals are typically not applied to the sensor. The normal sensor output provides a baseline for comparison to a sensor offset error detection signal ( 408 ) produced when a test signal may be applied to one self test electrode, and also to a sense error detection signal ( 406 ) produced when a test signal may be applied to both self test electrodes ( 208, 210 ).

BACKGROUND

1. Field

This disclosure relates generally to integrated sensors, and morespecifically, to testing and calibration.

2. Related Art

Inertial sensors may be used in many applications. They are typicallyused to detect force in one or more directions, typically forces createdby moving objects such as an airplane, train, vehicle or the like. Inshort, anything that has a mass that can be moved to create inertialforces can typically be measured by inertial sensors. An exampleapplication of inertial sensors, is in an automotive safety systems suchas air bag systems, anti lock-brakes (“ABS”), vehicle stability controls(“VSC”) and the like.

To integrate easily into an electrical system, such as found in avehicle, inertial sensors may be in the form of electronic circuits. Insuch circuits, the sensor (or transducer) can be a part of a multichipand sometimes integrated solution in a package. In such electroniccircuits, the electrical signal representing the force or motion may beproduced by any number of methods. Typically the force measured cancause a change in some fundamental electrical parameter of theintegrated circuit, such as capacitance, resistance, transistor gain,inductance, or the like.

Since movement can often occur in such circuits, as well asmanufacturing variations, the sensors outputs may vary greatly, or driftduring operation. In addition, mechanical forces being measured can bemade up of a spectrum of mechanical frequencies. Transducers may respondto the frequencies that make up such a mechanical input signaldifferently from sensor to sensor. Thus, a transducer may also have aunique frequency response that may also change over time. And also, agiven sensor may respond differently or fail after being used for awhile. As inertial sensing applications continue to grow, the demandsmade upon inertial sensors will most likely call for improved and morereliable sensors, and sensor performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited by the accompanying figures, in which like references indicatesimilar elements. Elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale.

FIG. 1 shows a sensing system including an inertial sensor withcontinuous self test.

FIG. 2 is a diagram showing the sensing element of an inertial sensorwith continuous self test.

FIG. 3 is a block diagram of an inertial sensor with continuous selftest.

FIG. 4 is a timing diagram showing the relationship between self testvoltages and error detection phases applied in a method of implementingcontinuous self test.

FIG. 5 is a flow diagram showing a process for continuous self test ofinertial sensors that includes three phases.

FIG. 6 is a flow diagram showing the first phase process, or normaloperating mode for continuous self test of inertial sensors.

FIG. 7 is a flow diagram showing the second phase process, or senseerror detection, for continuous self test of inertial sensors.

FIG. 8 is a flow diagram showing the third phase process, or offseterror detection, for continuous self test of inertial sensors.

FIG. 9 is a graph showing transducer spring constant changes occurringin the normal operation mode, the sense error detect mode, and theoffset error detect mode.

DETAILED DESCRIPTION

Current self test is usually performed at startup, because the sensor isusually not expected to detect an application event when theelectrostatic force is engaged and provisions are typically not made toattempt testing at a later time due to the possible presence of aninterfering operational signal. With current self test techniques,electrostatic forces may be applied to the transducer during normaloperation, which result in an offset at the sensor output. However,without proper techniques, and analysis, this method can causedistortion which can trigger an incorrect action initiated by thesensor. In addition, self test during normal operation with asuperimposed stimulus latch-up of the sensor can occur, which can beseen as a catastrophic failure at the device output.

The application of a new continuous self test (“CST”) to a sensor canallow the sensor (equivalently termed a device, a transducer, anaccelerometer, a g-cell micro-electro-mechanical systems sensor“g-cell”, or the like), such as an exemplary accelerometer, to be testedafter start up, and while it may be in use. As used in this document“continuous” as used in “continuous self test” may mean that one or moreself test process may be applied over continuous operation of thesensor, typically over a plurality of operational periods (typicallyafter start up and during use of the sensor) in which the sensor can beactive. Thus, self test is “continuous” in that it may be applied at anytime during operation of the sensor, and the self test processes neednot be applied constantly during, the operation of the sensor. Likewise,while an individual self test procedure, or procedures, can be appliedduring such continual operation of the sensor may not necessarily be“continuous” or “continuously applied during a particular process.Individual continuous self test processes may be of a digital or samplednature, and the various self test processes need not be appliedimmediately after each other. Individual processes may be applied atvarious times over the continuous operation of the sensor and may behave discrete timing, which can be asynchronous, clocked, or the like.CST can include modification of the sensor in conjunction withexercising the sensor, typically under the control of a processor, orits equivalent. In the examples described, a sequence of phases can beestablished to stimulate the sensor that can allow the detection of anerror in a transducer while maintaining the normal operation of thesensor, independent of input signal. With the addition of two detectionphases (in addition to a normal operating phase) a continuous self testoperation mode can allow detection of transducer element errors througha large range of forces (“g's”) and substantially any frequency,including 0 Hz. The term g's, generally refer to the measurement of anobject's acceleration, or the force that may be impressed on anaccelerating object. Such forces may be timed g's, g-force, g-load, orthe like. The examples below can be applied in sensing applications thatallow electrostatic actuation of the transducer element and the like.Such force detecting sensors may include micro-mechanical detectingelements that may be stimulated, or otherwise exercised by theapplication of test signals to specially supplied electrodes.

Sensors and accelerometers that can include continuous self test may befabricated by conventional micro-electro-mechanical systems (“MEMS”)techniques, or other equivalent methods. Sensors fabricated by the MEMStechnique to include continuous self test may be termed MEMS sensorshaving CST. In particular, the portion of the device that may includeforce sensing elements may be termed the g-cell. MEMS sensors includingthose with CST may be used in automotive electronics, medical equipment,smart portable electronics (such as cell phones, PDAS, hard disk drives,computer peripherals, wireless devices), and the like. In particular,MEMS (and other) sensors with CST may be used in automotive, medical,and other applications or devices where reliability may be of concern.

FIG. 1 shows a sensing system 100 including an inertial sensor withcontinuous self test 101. The inertial sensor with continuous self test101 typically includes circuitry for implementing one or more self testprocesses, and may include control circuitry constructed to execute orpartially execute, the processes described below. Exemplary sensingsystems 100 may include those for deploying air bags, systems fordetecting changes in wheel speed, and the like. In the general systemdescribed 100, there may be an environment to be controlled 114. Theinertial sensor (or equivalently any measurement device that can useelectrostatic voltages to provide a measurement) with continuous selftest 101 may be coupled to the environment to be controlled 114 in orderto receive stimulus signals from it.

The inertial sensor with continuous self test 101 may include a sensingelement 106 such as an exemplary, pressure sensor, accelerometer,inertial sensor, or g-cell operating in cooperation with a processingdevice which may include an exemplary ASIC 104, other associated signalprocessing circuitry, processor or the like. Such an inertial sensorwith CST 101 may be assembled as an integrated circuit (“IC”), hybridcircuit, printed wiring assembly, or the like. An exemplary sensor 101can be provided as an IC disposed in a conventional small outlineintegrated circuit (“SOIC’) package, or the like. The inertial sensorwith continuous self test 101 may provide a self test of the g-cell 106even while under the influence of the environment to be controlled 114after initial start up.

The sensor 106 typically includes electro-mechanical assemblies (g-cellor the like) and electronic circuitry that may be included with thesensor. The processing device 104 may be conventionally constructed toimplement CST and other signal processing, or conditioning functions.

The inertial sensor with continuous self test 101 may also be coupled toadditional control circuitry that may include off chip processingprovided by a processor, or its equivalent 108, working in cooperationwith a memory 110. In alternative examples the circuit functionsprovided by the processor 108 and memory 110 may be included in theinertial sensor with continuous self test 101 provided the ASIC 104 issuitably configured. In further alternative examples, analog or otherdigital circuitry may be equivalently substituted for the processor andmemory. The processor 108 typically receives data from the inertialsensor with continuous self test 101, and can process the data forsuitable application to a control circuit 112 coupled to the processor108. The inertial sensor with CST 101 may be provided as an IC disposedin a module. The printed wiring board (“PWB”) or assembly 118 mayinclude circuit functions suitable for interfacing with an inertialsensor with CST to implement functions such as controlling or activatingCST functions (or modes of operation), airbag deployment, or the like.

Processing may be aided by a software program implementing or otherwisecontrolling application of the CST method 116 and running on theprocessor 108. The processor 108 can be used to coordinate and executethe overall CST process, while some localized processing or execution offunctions may be provided within the internal sensor with continuousself test (ASIC 104). Alternatively, the CST method may be executed bythe control circuit constructed as a dedicated processing element suchas a DSP or the like. The processor 108 and memory 110 can beconventionally constructed. The control circuit 112 conditions theoutput of processor 108 coupled to it to produce a signal, or signals,suitable to control the environment 114 and the inertial sensor with CST101. Control of the environment may include verification of sensoroperation prior to actuating the control circuit 112, such as actuatingan airbag system.

An example of such a system 100 may be the use of MEMS sensors with CST101 in automotive air bag safety systems. Here, an end user producing anairbag system, including a MEMS sensor with CST may wish to providefailsafing, or other reliability measures to ensure proper operation ofthe air bags. An IC typically providing CST outputs/inputs 101 may beused in conjunction with additional circuitry, typically on a PWB 118,to provide failsafing and other features.

Customers that may use automotive inertial sensors may wish to includefailsafing in a system 100. Failsafing is generally a term that canrefer to redundancy of components, or to detection of faults. Thecurrent self test techniques available typically do not allow continuousmonitoring of the transducer element after startup. A MEMS sensor withCST 101 may allow for the provision of failsafing in a sensing system100 having a sensor with continuous self test 101, after start up. Aspart of a comprehensive failsafing system, continuous self test may beprovided in automotive inertial sensor applications and the like.Failsafing may be provided by circuitry constructed to utilize thecontinuous self test capabilities available from the sensing element106.

Continuing with the exemplary airbag system 100, the environment to becontrolled 114, may be an automobile crash, where the inertial sensingsystems 101 detects a change in acceleration indicative of a crash orother event. Output of the sensing element 106, working in cooperationwithin the ASIC 104, tends to be without false triggering due to theprovision of CST. CST ensures that the sensor is working prior to thecrash event. In other applications such as rollover detection CST may beused in real time during the event. The output of the inertial sensor101 may be applied to processor 108, and memory 110 for processing andapplication to the control circuit 112. The processor 108 may addressthe inertial sensor with CST 101, by cycling the phases applied to theinertial sensor 101, and evaluating the output. In one example thecontrol circuit 112 is can be part of an air bag assembly that canoperate under control of the processor 108 to deploy the airbags toprotect those in the vehicle. With CST being part of the inertial sensor101 more reliable and accurate deployment of the airbags in an airbagsafety system 100 can be provided.

An exemplary MEMS sensing element including continuous self test 106 maybe fabricated by surface micro-machining. In surface micromachining, theMEMS sensor can be conventionally formed on top of a wafer with thinfilm materials disposed thereon. The top layers may consist ofstructural materials forming the sensor and sacrificial layers that canbe used to define gaps between the structural layers. The remainingmaterial may move, allowing motion to be detected. Many surfacemicro-machined sensors use a capacitive transduction method to convertan input mechanical signal (motion or acceleration) to an equivalentelectrical signal. In the capacitive transduction method, the sensor canbe considered to be a mechanical capacitor in which one or more of theplate's moves with respect to the applied physical stimulus. The changein capacitance can thus, be considered the electrical equivalent of theinput mechanical stimulus. The provision of additional plates orelectrodes in the sensing element 106 can provide electrical signals foruse in providing CST. The sense plates themselves also provide for CST,but with a different front end.

FIG. 2 is a diagram showing the sensing element 106 of an inertialsensor with continuous self test 201. The inertial CST sensor electrodearrangement 201 can allow for providing continuous self test by anintegrated circuit. Inertial sensors 106 may be used to detect motion inone or more directions, such as exemplary linear accelerations in x, y,or z directions. Accordingly, the sensors may be configured to measureforces in one or more directions. And, CST sensor electrodes may beprovided in each direction. Sensors 106 with CST 201 used in suchapplications may typically be provided as integrated circuits in aneffort to improve reliability, performance, economy, and the like. Theg-cell may include one or more moveable capacitor plates 202 to measureg-force, fixed electrodes for self test 208, 210 and sensor plates 212,214. Coupling of the electrodes to an external circuit (typically via anASIC) typically allows passage of conventional signals but also allowsaccess to the plates and electrodes 208, 210, 212, 214. The inertialsensor assembly with CST 106 may be disposed in a conventionalintegrated circuit package and it may be electrically coupled toexternal circuitry such as a PWB (118 of FIG. 1), typically through aconventional techniques, so that self test may be controlled.

In an exemplary implementation of a surface micro-machined integratedcircuit accelerometer, silicon, or an equivalent material, may beselectively etched to form fixed electrodes 208, 210 with a movableelectrode 202 between. Movement can cause a displacement of the movableelectrode, causing a change of capacitance in the two back to backcapacitors formed (208 and 202, 210 and 202) in this electrodearrangement. This plate arrangement provides signals indicative of anapplied g-force.

When the center plate 202 deflects, the distance from it to one of thefixed plates 208, 210 can increase by the same amount that the distanceto the other plate decreases. The change in distance is a measure ofacceleration. As the center plate moves when sensing acceleration, thedistance between the plates changes, and each capacitor's value willchange, (C=Aε/D). Where A is the area of the plate, ε is the dielectricconstant, and D is the distance between the plates. In alternativeexamples there may be uni-directional and/or bi-directional self testplates for use in implementing CST. Self test plates 208, 210 areprovided for CST and may be termed an offset error self test plate, orelectrode, and a sense error self test plate or electrode. The platescan be one or more additional plates provided that can establish acapacitance between the middle or common plate 202 and the self testplates 208, 210. The provision of one self test plate is an example of aunidirectional self test plates. In an alternative bi-directionalexample having two self test plates for a sense mode of CST, there maybe a positive self test plate, and a negative self test plate.

MEMs sensors with continuous self-test plates 208, 210 can provideverification of the mechanical and electrical integrity of theaccelerometer at any time, before or after, installation. Continuousself test may be useful in heightened reliability applications such as,the exemplary automotive airbag system where system integrity can beensured over the life of the vehicle.

When an exemplary logic high is input to the self test pin of the sensorIC or equivalent digital mode in the sensor IC is entered, a calibratedpotential can be applied across the self-test plate (208 or 210) and themoveable plate 202. The resulting electrostatic force can cause thecenter plate 202 to deflect. The resultant deflection is measured by theaccelerometer's control ASIC, (104 of FIG. 1), as a proportional outputvoltage results. This procedure can assure that both the mechanical(g-cell) and electronic sections of the inertial sensor (101 of FIG. 1)are functioning. In an alternative embodiment of continuous self testplates, the fourth plate (208 or 210) may be omitted, as the self testplate (208 or 210) and sense plate (212 or 214) functions may becombined with only minor modifications made to the appropriate systemsensor transfer function to account for this combination. Othercombinations of stimuli, or phases, can be applied to one or more selftest plates to produce various responses to implement CST. Inparticular, the normal phase, the error detection phase, and its offseterror detection shown may be provided.

FIG. 3 is a block diagram showing further detail of an ASIC in aninertial sensor with continuous self test. The diagram shows theoperation of the g-cell 106, in conjunction with the ASIC 104. The ASICis a dedicated processor, typically implemented in hardware to providespecific processing functions which may include various circuitfunctions 302, 304, 308, 310, 312, 314, 316, 318, 320 that can beprovided in CMOS or other equivalent circuit implementations. The ASICcan provide dedicated circuit processing of the CST signals from theg-cell 106, as well as providing CST stimulus signals 302, 304 to theg-cell 106 according to the test phases typically applied underdirection of an external controller (108 of FIG. 1). An externalcontroller can provide control flexibility as it may be programmed withsoftware, while the ASIC, or dedicated processor, may provide morededicated processing with limited memory (as often provided by storageregisters). In the example provided control and implementation of aprocess for providing continuous self test can be shared to varyingdegrees between the ASIC and an external controller with distribution ofprocessing typically decided by typical engineering considerations suchas a trade off of speed for flexibility, or the like. However, thisimplementation is not intended to be limiting. In alternative examplesall of the processing may be provided on chip in an ASIC, ASIC/onboardprocessor, or the like. In a further alternative example all of theprocessing may be provided off of a chip by a computer, microprocessoror the like typically being directed by conventional programmingtechniques.

Test stimulus to the g-cell 106 can be provided by coupling test signalsVst1 302 and Vst 2 304 to the g-cell 106. Test signals 302, 304 aretypically analog voltages selected to provide a desired result whenapplied during CST phases. Pulse Width Modulated input signals can alsobe used, alternatively, digital signals can be used as well. Both typesof signals may represent an equivalent voltage to the sensor. Thevoltages 302, 304 may be externally supplied through an IC pin (ballgrid array pad or the like), or may be generated internally byconventional methods, typically in response to a logic or similarcontrol signals applied to IC pins.

The g-cell element 106 may be a conventionally formed MEMS sensor orequivalent constructed by various processes and materials includingCMOS, poly-silicon, and the like to include the CST electrodes (202,208, 210, 212, 214 of FIG. 2). An exemplary sensor that may include CSTelectrodes is the exemplary Micro-Electro Mechanical Systems (“MEMS”)device that may be formed by micro-machining, or equivalent processes.The exemplary MEMS device consists of a surface micro machinedcapacitive sensing element 106 and a CMOS signal conditioning ASIC 104of FIG. 1) usually contained in a single integrated circuit package.

Inertial sensors operation may be arbitrarily divided into variousforce, or “g” categories such as low, medium, high and the likedepending upon the amount of force to be measured. The CST electrodearrangement may be utilized in each of these and other categories.

The CST calculation block 320 can be a conventionally constructedprocessing circuit for controlling operation of the inertial sensor ICand processing the CST signals and transfer functions. The CSTcalculation block may be coupled to the Sinc+LP block 310, and theregister array block 316.

In the present example a sigma-delta (“ΣΔ”) conversion 308, can be atype of analog-to-digital or conversion characterized by integrating(i.e., Σ) differences (i.e., Δ) by conventional methods. Here,capacitance received from block 106 can be converted to a digital outputproportional to the capacitance. The output of the converter 308 may becalled a count which is produced by a digitalization of the inputvoltage from the g-cell. In alternative examples equivalent conversioncircuits may be substituted for the sigma-delta conversion.

The sinc filter (“Sinc+LPF”) 310 can be a filter that can removefrequency components above a given bandwidth, leaves the low frequenciesunaffected, possesses linear phase and the like. The sinc filter isconventionally constructed, and coupled to the sigma-delta conversionblock 308.

The compensation block 312 can be conventionally constructed andtypically compensates the raw output of the sensor 106. The compensationblock may be coupled to the Sin+LP block 310. The compensation istypically provided by trimming of the g-cell for static errors such assensitivity, offset, linearity, and temperature variation. Thecompensation provided is usually specific to trimming the data outputtedfrom the g-cell and to make up for the variances that can be found inthe signal path. This block can be provided since the performance of theg-cell typically cannot meet product specifications withoutcompensation. However, in alternative examples this block may be omittedfor precision g-cells not needing compensation, or for typical g-cellsthat have looser specifications.

The low pass filter (“LPF”) 314 may be provided as a conventional 4-poleswitched capacitor low pass filter 314 or the like. The LPF 314 can becoupled to the compensation block 312. The LPF 314 may be used to createthe proper precision for digital calculations (i.e., noise reduction).Bandwidth of the LPF 314 may be dictated according to the sample rate ofthe phases. A Bessel implementation or equivalent can be used because ittypically provides a maximally flat response with linear phase. Thus,this filter 314 may tend to preserve pulse shape integrity. Because theexemplary filter 314 is constructed using switched capacitor, or digitaldesign techniques, there is typically no need for external passivecomponents (resistors and capacitors) to set the cut-off frequency.

The LPF 314 is typically provided to reduce the noise in the bit stream.Generally speaking, the only time that a bit stream is present is out ofthe sigma delta converter 308. The sinc 310 turns the bit stream into aparallel n bit value. No longer a bit stream, but still a digital valuethe low pass filter 314 can reduce a wide band signal, typically one upto about 10 kHz, and cut it back by about 400 Hz to 50 Hz depending onthe application.

A cut off of 50 Hz is an arbitrary or exemplary value, as every customertypically has a different value specified for their use. Cut off, mayalso depend on the sensor sensitivity. In a low g-force verses a mediumg-force application or more importantly, in an air bag verses anelectronics stability application sensitivity specifications vary.Airbag applications can use 400 Hz within a variance of approximately 10to 20 Hz depending on the customer. Electronic stability application,such as rollover detection in a car, would typically utilize a 50 Hz cutoff.

The register array 316 typically includes a plurality of registers (CSTSense, CST_Off, and the like) used to store data. The register array maybe coupled to the LPF 314 and the CST Calc block 320. Information fromthe CST calculation block 320 may be stored here for later use. Also,the data in the plurality of registers may be accessed by a user throughdigital communication (but this is not required for CST implementation)the SPIE 318. The register array may be conventionally constructed.

The serial programmable interface (“SPI”) 318 is an exemplary interfacecircuit that allows an external control circuit (118 of FIG. 1) accessto sensor data typically by conditioning an output of the sensingelement coupled to it for presentation to a data interface The SPIinterface is conventionally constructed and may be coupled to theregister array 316. Other interfaces may be provided in alternativeexamples.

The signal path shown is exemplary; the circuits described above canalso be implemented as an analog circuit signal path or the like. Inalternative examples processing by other signal processing pathconfigurations are possible too. The signal processing path describedabove is exemplary only and for a single sensor. Sensing systems (100 ofFIG. 1) can have multiple sensors with continuous self test (101 of FIG.1), and may sense in one or more dimensions, with continuous self testprovided for each dimension being sensed.

FIG. 4 is a timing diagram 400 showing the relationship between selftest voltages 402, 404 and error detection phases 406, 408 applied in amethod of implementing continuous self test typically in conjunctionwith a normal operations mode 412. The timing diagram 400 shows themutually exclusive operation of normal operation 418 and CST operationmode 420. The normal operation phase 418 typically has stimuli andresulting signals that may originate from g-forces applied to thetransducer. Continuous self test (CST) operation can include phases 406,408 to apply a voltage or voltages 402, 404 to the self test plates.Bi-directional versus uni-directional CST operation is typicallydependent on the detection phase request made to the device. Thedetection phase request is typically configurable through a 4 bitcontrol register, CST_config that is part of the register array (316 ofFIG. 3).

The timing and application of the signals shown can be provided by anexternal processor (108 of FIG. 1). Establishing a sequence of phasescan allow the detection of an error in the transducer (101 of FIG. 1),while maintaining normal operation of the transducer by initiating CSToperation 408. The two detection phases 406, 408 shown occurring duringthe continuous self test (CST) operation mode 420 can allow detection oftransducer element errors at typically any g-force and any frequency,including 0 Hz. Thus, no stimulus or signal mode may be left where CSTconflicts with the external signal, which can provide true failsafeoperation.

In normal operation 412, the self test plate (708, 210 of FIG. 2)voltages, (Vst1 and Vst2) voltages may be maintained at the samepotential (or zero potential) as the middle plate (202 of FIG. 2), toproduce no effect on the sensor output. In normal operation, this modemay be cycled any number of times (or n cycles) 418 as requested by thecontroller (108 of FIG. 1). Thus, the device may be run in normaloperation mode continuously or up to any number of times.

CST operation 410 may be initiated at any time. Typically during normaloperation 412, the CST operation mode may be invoked for m cycles 420.The controller can decide how many cycles may be provided. The number ofcycles 420 could be from one to infinity depending upon the systemspecification or user preference.

When in CST Op mode 410, two errors can be detected, one is an offseterror 408 and the other is a sensitivity error 406. The order ofdetermination of these errors 406, 408 may be interchanged. As shown inthe exemplary timing diagram 400, the offset error detection 408 can bedone prior to sense error detection 406.

In performing offset error detection 408, a voltage can be applied toone of the self test plates 414, to pull the middle plate (202 of FIG.2) towards one of the sense plates (208 or 210 of FIG. 2), eitherpositive or negative as desired. In a MEMS device, there is typically asquare law relationship of voltage, to its distance that the middleplate will travel in distance. Thus, if the middle plate is made to movecloser to the sense plate, an increase in capacitance can result. Thesensor signal processing path can process the offset signal applied asan increase in capacitive output which can be read by the externalcontroller. Thus, this test may allow an offset, or shift in capacitancefrom the sensor to be read by the controller. If an offset is not seen,that may indicate a failure in the sensor.

Sense error detection 406 can be performed when offset error detection408 is not being performed. Here, a voltage 402, 404 can be applied toboth self test plates (208 and 210 of FIG. 2). When equal potentials areapplied the middle plate (202 of FIG. 2) will not move. However, thespring constant of the MEMS device will have been changed by applicationof the voltages 402, 404. Typically, the MEMS capacitor is made moresensitive by applying the voltages. The CST Calc Block (320 of FIG. 3)would see a change at the output of the sensor since the gain of theelectronic signal path has not changed. For example, if the output wasat a count of 300, a jump to a count of 310 or 320 or similar may beread, depending on how much voltage is applied to the plates.

As used in the application, a count is what the customer sees at theoutput of the digital signal processing chain, typically provided by theASIC. When applying voltages to both plates Vst1 and Vst2, the effectiveg-cell spring constant changes, so an output in capacitance from theg-cell tends to increase. So when the self test detection processes areperformed (either the offset or the sense), the output of the sensorchip changes. It is assumed that no stimulus (g-shock) has been appliedto the g-cell. In other words, the g-cell is in normal operation mode.If a stimulus was applied a change at the output would typically beseen. Flexibility in applying CST is provided so the customer can evokethe detection mode any time. Alternatively a customer may have anindependent algorithm, that figures out the proper time to evoke thisdetection in mode.

Generally speaking, the customer typically does not want the phases tobe applied when a g-force stimulus is applied, because raw output,without the g-cell put in some other mode besides normal operation,tends to be most useful. So the m cycles, will typically be less thanthe n cycles shown. Usually for systems that would use this type ofsensor with CST an update rate of about 1 millisecond may typically beran. And the m cycle should be as short as possible.

Signals Vst1 & Vst2, can be applied either separately or in combination.A stimulus (Vst1 or offset error test voltage) is applied to the oneplate for a given time to perform offset error testing 408 and stimulusVst2, or sense error test voltage, is typically then applied for aportion of the time that Vst1 is applied to perform sense error testing406.

FIG. 5 is a flow diagram showing a process for continuous self test ofinertial sensors that includes three phases. The three phases or processfor continuous self test of inertial sensors include: a normal operationprocess 502, a sense error detection process 510, and an offset errordetection process 512. The processes may be applied any number of timesduring operation, and in any combination.

Initially the transducer may be operated in the normal operating mode502. Next the mode of operation may be changed by disabling the normaloperating mode 504. The next mode of operation may be the CST operationmode 516. The CST operation mode may include operation in offset errordetection mode 508, and operation in sense error detect mode 512. Offseterror detection is enabled 506, and then performed 508. During this testoffset error detection 506 may be performed one or more times. Nextsense error detection is enabled 510, and performed 512. During thistest sense error detection 512 may be performed one or more times. Andfinally the CST operation mode is disabled 514, and the process may berepeated by returning flow control to operation in the normal operationmode 502.

The process may repeat any number of times and may be performed in anyorder. In alternative examples CST operation may be performed first. Infurther alternative examples either offset error detection 508, or senseerror detection 512 may be performed individually and without the othertest. The provision of CST in the examples shown allow a high degree offlexibility for a customer, or user buying the inertial sensors (101 ofFIG. 1) to configure the number of times a test is run, and which tests,and what order to run the tests. In addition each test may be ran aplurality of times, before proceeding to the next test which may be rana second plurality of times.

FIG. 6 is a flow diagram showing the first phase process, or normaloperating mode for continuous self test of inertial sensors 502. Atblock 602 the transducer output can be monitored. Next at block 604output samples from the transducer are collected. At block 606 thesamples may be presented or coupled to the CST transfer block to createa signal reference, or baseline.

In the normal operation phase, or mode, (“Norm Op phase”) 502 the sensoroutput can be monitored. The normal operational output can be directedthrough the main signal processing path of the inertial sensor (101 ofFIG. 1) where samples can be stored in a register. The register can beread by a processor (108 of FIG. 1) or other equivalent circuitrythrough the SPI (or equivalent) interface. The processor (108 of FIG. 1)can access this register to use the normal operating sensor output as aninput variable (Out Norm Op) for later use in evaluating sensor transferfunctions.

FIG. 7 is a flow diagram showing the second phase or process, for senseerror detection, for continuous self test of inertial sensors 508.First, sense error detection is enabled 702. Next an electrostatic forcecan be applied to each side of the transducer self test plates (208, 210of FIG. 2) 704. At block 706 an output is created for the transducerwith the gain change. And finally the sense error detection output canbe evaluated as compared to the normal operating output 708. Comparisonmay be provided by evaluating equation (1).

The sensor error detection phase (“Sense Err Detect Phase”) is the realtime error (or failure) detection phase. An electrostatic force can beapplied to both sides of the transducer self test plates to create again change, by electrically changing the effective spring constant ofthe element. The sensor output can be stored in the register array andprocessed in the CST block transfer function as described by equation(1). This output is returned to the register array in the CST_Senseregister for access by the customer. If the value is nonzero an errorhas occurred. Changing the electrostatic spring constant in the SenseErr Detect phase, can allow the detection of a change in the sensitivityof the device independent of the stimulus applied to the device. The “k”terms are somewhat arbitrary, as they may depend upon the g-cell, orMEMS structure.

CST_Sense=Out_(Norm OP)−Out_(Sense Err Detect)*LP_(adjust)*(k _(change)+k _(change) ²)  (1)

FIG. 8 is a flow diagram showing the third phase or process, for offseterror detection 505, for the continuous self test of inertial sensors.First the offset error detection process can be enabled 802. Next atblock 804 an offset change can be created by applying an electrostaticforce to one side of a CST plate. The side stimulated by the CST plateis immaterial. Typically one side, or the other is, stimulated. However,in alternative examples stimulation may be alternated between sides aslong as the differing offsets, and other calibration factors that takeinto account the usage of one plate over the other are accounted for. Atblock 806 the offset is determined. And finally at block 808 the offseterror can be compared to the normal operation reference previouslystored via utilization of equation (2).

The offset error detection phase (“Off Err Detect phase”) is a delayeddetection phase, averaged over an arbitrarily chosen number of m cycles.An electrostatic force can be applied to one side of the transducer tocreate an offset change. The sensor output is processed as described byequation (2). The processor output is stored in a register for access bythe customer at register location CST_Off. If the value is nonzero anerror has occurred.

CST_Off=Out_(Norm Op)−Out_(Off Err Detect)+Off_(change)  (2)

The Off Err Detect phase is provided because the Sense Err Detect phase(that biases both plates to change the spring constant) typically doesnot work for a small window of g's where the sensor gain change cannotbe resolved. This window is typically around a natural offset of thetransducer element (106 of FIG. 1). The Off Err Detect phase (one platebiased to create an offset) typically cannot solely be used becausethere is the possibility that an external stimulus can be applied to thetransducer matching the applied test stimulus. Another example may be aslow averaging integration, which typically does not allow 0 Hzdetection.

Equation (1) and (2) can represent exemplary transfer functions of theCST process. The CST transfer functions can be evaluated after eacherror detect phase has been set up and applied. The registers can beupdated each time new data is available. At any time the customer canvalidate the behavior of the transducer element of this sensor byexamining the CST registers. If no externally applied event (applicationof g-force) is occurring the customer would typically read the CST_Offregister after m periods. If a force application event occurs theCST_Sense register would be read. The equations above may includeseveral correction factors or terms, so that the transfer functions mayyield the desired results.

Transfer function CST_Sense (equation (1)) can include correctionfactors. Due to process variations, test gauge error, and signalprocessing inaccuracies an error specification can be established foreach CST transfer function being evaluated. In the normal behavior ofthe transducer when a voltage is applied to the plate the voltage willcause a distortion in the plate inversely proportional to the gapbetween plates. The correction factor for the slope in the CST_Sensetransfer function can be represented by k_(change). The CST_Sensetransfer function can also adjusted for the roll off of the low passfilter by the term “LP_(adjust)”. The LP_(adjust) factor may bedetermined at trim, usually indirectly through oscillator trim, or bydirect measurement. In oscillator trim, or trim by direct measurement,electronic trim can be provided by a fuse bit. A conventional erroranalysis of the signal path can determine the appropriate method.

LP_(adjust) and Off_(change) compensate for similar manufacturingvariations. Here, filter roll off changes from g cell to g cell soLP_(adjust) is supplied to account for each g cell's own variations andaccount for that variation in the equation to provide a zero for aresult when the equation is evaluated.

Transfer function CST_Off can be corrected by the expected offset delta,Off_(change). When there is a difference, or delta, between the normaloutput and the offset error detect output, a known delta can beobtained. The known delta can be stored in the register, and that's thefactor Off_(change). Thus, CST_Off should equal zero if a part isfunctioning properly when the factor Off_(change) is indicated in thetransfer functions. If the g-cell has a problem, when in use, theCST_Off will not equal zero, indicating to the system that there is aproblem with the sensor. Off_(change) is included to account for processvariation and the like, that can cause each g-cell to be slightlydifferent from others. The difference is typically determined fromg-cell to g-cell and stored for use in evaluating the transferfunctions. The operation of the phases in providing CST may be betterunderstood by considering the following example.

FIG. 9 is a graph showing typical transducer spring constant changesoccurring in the normal operation mode 910, the sense error detect mode912, and the offset error detect mode 914. The horizontal axis isg-level, and the vertical axis shows peak amplitude of voltage oralternatively output capacitance of the g-cell.

The Off Err Detect phase is provided because the Sense Err Detect phaseonly detects an error outside a certain window 902. As shown, there issome point 918 where spring constants k1 and k2 cross. This crossingpoint 918 can be dependent on the natural offset of the movable platedue to process, temperature and package variation. For no variation thepivot point 918 may be at 0 g's. The pivot point 918 can move overtemperature, but the time constant is typically slow and can bespecified for each application.

The smallest gain error window is typically specified based on theindividual variation of the devices, and test data. The variation shouldbe kept to a minimum to maximize the real time detection area for thegiven application.

The following process identifies the behavior of the g-cell relative toresults of equation 1 and equation 2:

CST_Sense ≧ Err_(spec);(CST_flag= 1) else if   |average(CST_Off_(nperiods))|≧Err_(spec);(CST_flag = 1) else CST_flag = 0  CST_flag = 1    bad g-cell   CST_flag = 0    good g-cell.

The examples above provide:

1. A system comprising:

a sensing element having a continuous self test capability; and

-   -   a processing device directing continuous self test, coupled to        the sensing element for providing the continuous self test of        the system being performed during normal operation of the        system, when g-forces are not present.        2. The system of claim 1 in which normal operation of the sensor        is disabled when self test is actuated.        3. The system of claim 1, in which continuous self test occurs        in at least one time period of a plurality of time periods        during normal operation of the sensor.        4. The system of claim 1, in which the sensing element is an        inertial sensor.        5. The system of claim 1, in which the sensing element is a MEMS        device having a movable electrode disposed between a pair of        fixed electrodes.        6. The system of claim 1, in which the processing device is a        dedicated processor.        7. The system of claim 1, in which the processing device is an        ASIC.        8. The system of claim 7, in which the processing device        includes an interface circuit to format an output of the        processing device for presentation to a data interface.        9. The system of claim 1, in which the processing device further        includes an offset error generation circuit, coupled to the        sensing element, to generate an offset error test voltage.        10. The system of claim 9, in which the processing device        further comprises a sense error generation circuit coupled to        the sensing element to generate a sense error test voltage.        11. An inertial sensor circuit comprising:    -   a sensing element having a movable electrode disposed between a        first electrode and a second electrode of a pair of fixed        electrodes providing a continuous self test capability; and    -   a processing device controlling continuous self test, coupled to        the sensing element for directing and providing stimuli for the        continuous self test of the system performed at any time while        the inertial sensor circuit is operated under normal conditions        and, when g-forces are not present.        12. The inertial sensor circuit of claim 11 in which an offset        error test signal is coupled to the first electrode to create an        offset change output from the sensing element.        13. The inertial sensor circuit of claim 12 in a sensor error        test signal is coupled to the second electrode to create a gain        change output from the sensing element.        14. The inertial sensor circuit of claim 13 in which the        processing device includes a register array for comparing a        previously recorded reference signal to the offset change        output.        15. The inertial sensor circuit of claim 13 in which the        processing device includes a register array for comparing a        previously recorded reference signal to the gain change output.        16. A method of providing continuous self test of an inertial        sensor comprising:

operating the sensor in a normal operating mode;

inhibiting the normal operating mode; and

-   -   operating the sensor in a continuous self test mode at any time        during operation of the sensor.        17. The method of providing continuous self test of a sensor of        claim 16, in which operating the sensor in continuous self test        mode includes creating a reference.        18. The method of providing continuous self test of a sensor of        claim 16, in which the continuous self test mode includes        operating the sensor in an offset error detection mode.        19. The method of providing continuous self test of a sensor of        claim 16, in which the continuous self test mode includes        operating the sensor in a sense error detection mode.        20. The method of providing continuous self test of a sensor of        claim 16, further comprising comparing an output from the sensor        operating during the offset error detection mode and an output        from the sensor operating in the sense error detection mode to a        reference signal output from the sensor operating in the normal        operating mode.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope of thepresent invention. Any benefits, advantages, or solutions to problemsthat are described herein with regard to specific embodiments are notintended to be construed as a critical, required, or essential featureor element of any or all the claims.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

It is understood that all circuitry described herein may be implementedeither in silicon or another semiconductor material or alternatively bya software code representation of functions provided by silicon oranother semiconductor material.

1. A system comprising: a sensing element having a continuous self testcapability; and a processing device directing continuous self test,coupled to the sensing element for providing the continuous self test ofthe system being performed during normal operation of the system, wheng-forces are not present.
 2. The system of claim 1 in which normaloperation of the sensor is disabled when self test is actuated.
 3. Thesystem of claim 1, in which continuous self test occurs in at least onetime period of a plurality of time periods during normal operation ofthe sensor.
 4. The system of claim 1, in which the sensing element is aninertial sensor.
 5. The system of claim 1, in which the sensing elementis a MEMS device having a movable electrode disposed between a pair offixed electrodes.
 6. The system of claim 1, in which the processingdevice is a dedicated processor.
 7. The system of claim 1, in which theprocessing device is an ASIC.
 8. The system of claim 7, in which theprocessing device includes an interface circuit to format an output ofthe processing device for presentation to a data interface.
 9. Thesystem of claim 1, in which the processing device further includes anoffset error generation circuit, coupled to the sensing element, togenerate an offset error test voltage.
 10. The system of claim 9, inwhich the processing device further comprises a sense error generationcircuit coupled to the sensing element to generate a sense error testvoltage.
 11. An inertial sensor circuit comprising: a sensing elementhaving a movable electrode disposed between a first electrode and asecond electrode of a pair of fixed electrodes providing a continuousself test capability; and a processing device controlling continuousself test, coupled to the sensing element for directing and providingstimuli for the continuous self test of the system performed at any timewhile the inertial sensor circuit is operated under normal conditionsand, when g-forces are not present.
 12. The inertial sensor circuit ofclaim 11 in which an offset error test signal is coupled to the firstelectrode to create an offset change output from the sensing element.13. The inertial sensor circuit of claim 12 in a sensor error testsignal is coupled to the second electrode to create a gain change outputfrom the sensing element.
 14. The inertial sensor circuit of claim 13 inwhich the processing device includes a register array for comparing apreviously recorded reference signal to the offset change output. 15.The inertial sensor circuit of claim 13 in which the processing deviceincludes a register array for comparing a previously recorded referencesignal to the gain change output.
 16. A method of providing continuousself test of an inertial sensor comprising: operating the sensor in anormal operating mode; inhibiting the normal operating mode; andoperating the sensor in a continuous self test mode at any time duringoperation of the sensor.
 17. The method of providing continuous selftest of a sensor of claim 16, in which operating the sensor incontinuous self test mode includes creating a reference.
 18. The methodof providing continuous self test of a sensor of claim 16, in which thecontinuous self test mode includes operating the sensor in an offseterror detection mode.
 19. The method of providing continuous self testof a sensor of claim 16, in which the continuous self test mode includesoperating the sensor in a sense error detection mode.
 20. The method ofproviding continuous self test of a sensor of claim 16, furthercomprising comparing an output from the sensor operating during theoffset error detection mode and an output from the sensor operating inthe sense error detection mode to a reference signal output from thesensor operating in the normal operating mode.